The overall objective of the proposed research is to understand the molecular basis for the hypercalciuria of human idiopathic hypercalciuria (IH). Features of IH including normal serum CA, increased intestinal Ca absorption, and negative Ca balance and bone loss exaggerated by low Ca diet are also found in the genetic hypercalciuric stone-forming (GHS) rat, which is a unique animal model to study human hypercalciuria and nephrolithiasis. Like some patients with IH, GHS rats have normal serum 1,25 (OH)2D3 levels. Elevated vitamin D receptor (VDR) levels in intestine, kidney, bone, and splenic monocytes may mediate the hypercalciuria in GHS rats by exaggerating 1,25 (OH)2D3 biologic actions. Peripheral blood monocytes (PBMs) from IH patients overproduce cytokines (IL-1alpha and beta, IL-6), TNFalpha, and GM- CSF) that have osteoclast-stimulating activity. Pathologic increases in VDR and/or over-production of cytokines may result in the IH phenotype and both hypotheses will be tested in IH patients and GHS rats. The hypotheses will be tested in three specific aims: 1. That IH patients with normal serum 1,25 (OH)2D3 have excessive VDR and exaggerated vitamin D target tissue response to 1,25(OH)2D3 and that PBMs overproduce cytokines. Studies in IH patients will measure: a) dermal fibroblast and PBM VDR levels, 24-hydroxylase(24- OHase) activity, and PBM cytokine production; b) duodenal VDR levels and Ca absorption; and c) 1,25 (OH)2D3-dependent and independent bone resorption. 2. That VDR is constitutively increased in GHS rats by measuring VDR under conditions that modulate VDR levels in normal rats: a) decreased VDR as during aging, high Ca diet, parathyroid hormone; b) increased VDR as during low Ca diet, and low phosphate diet; and c) nephron co- localization of VDR, calbindin 28 kd, and Ca-sensing receptor. 3. Cytokine production in GHS rats: a) in vitro LPS- and 1,25 (OH)2D3- stimulated; b) during low Ca diet; c) effect of in vivo inhibition of IL- 1beta, TNFalpha action on urine Ca, intestinal Ca transport, and VDR levels; and d) effect of in vitro cytokines on monocyte VDR, and 24 - OHase activity. The proposed studies should lead to novel approaches to the treatment and prevention of hypercalciuria and Ca nephrolithiasis.